Method of constructing a casting mould by determination of isothermal pattern



Sept. 27, 1966 A. P. BANKS METHOD OF GONS'IRUCTING A CASTING MOULD BYDETERMINATION OF ISOTHERMAL PATTERN 2 Sheets-Sheet 1 Filed Aug. 24, 1964FIG. 1.

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METHOD OF CONSTRNCTING A CASTING MOULD BY DETERMINATION OF ISOTHERMALPATTERN Flled Aug. 24, 1964 2 Sheets-Sheet 2 FIG. 5.

United States Patent 3,274 652 METHOD OF CGNSTRUC'I ING A CASTING MOULDBY DETERMINATION OF ISOTHERMAL PAT- TERN Allan Pearson Banks,Workington, Cumberland, England, assignor to Distington EngineeringCompany Limited Filed Aug. 24, 1964, Ser. No. 391,808 Claims priority,application Great Britain, Aug. 23, 1963, 33,444/ 63 4 Claims. (Cl.22-493) This invention relates to the casting of metal and mouldstherefore.

It has been a constant problem hitherto to reduce the susceptibility ofsuch casting moulds to cracking during use, particularly cast ironmoulds for casting steel ingots. Various expedients have been proposedfor strengthening such moulds, for example steel bands encircling themould cavity and incorporated in the mould during formation of thelatter. Such expedients have not in general proved satisfactory for onereason or another.

Furthermore, during the construction of such moulds, there is anundesirable tendency to form thin films of high phosphorous metal at theinternal corners of the mould, a defect often encountered in theconstructions of conventional moulds, (known as corner shelling).

An object of the invention is the provision of a method of casting metalwhereby the walls surrounding the casting space are less likely to crackthan when previously proposed methods are employed.

Another object of the present invention is the provision of a castingmould which is less prone to crack than moulds proposed hitherto.

A further object is the provision of a method of constructing a mouldwhich reduces the incidence of corner shelling.

According to one feature of the invention, there is provided a method ofcasting metal in which molten metal is introduced into a moulding spacebounded by a wall the thickness of different parts of which is such thatthe passage of heat through the thickness of the wall during cooling ofthe molten metal is at a substantially constant rate throughout theWall.

According to another feature of the invention, there is provided acasting mould for carrying out the above method, the walls of which areshaped in cross-section so that on casting metal in the moulding spacethe crosssectional isothermal pattern during cooling consists of linesextending in the plane of said section substantially parallel to thesides of the moulding space.

According to a further feature of the invention, there is provided amethod of constructing a mould in accordance with the present invention,the mould having a moulding space or cavity of given cross-sectionalshape and dimensions, comprising the steps of:

(a) Determining the cross-sectional isothermal pattern which exists inthe Walls of a mould having walls of uniform thickness and having amoulding space or cavity of said given shape and dimensions duringcooling of metal cast in such mould; and p (b) Constructing a finalmould having a moulding space or cavity of said given shape anddimensions and having a wall the cross-sectional perimeter andperipheral shape of which conforms substantially to a selected one ofthe isotherms of said isothermal pattern.

3,274,652 Patented Sept. 27, 1966 According to yet a further feature ofthe invention, there is provided a method of constructing a mould havinga moulding space of given cross-sectional shape and dimensions,comprising the steps of:

(a) Determining the cross-sectional isothermal pattern which exists inthe walls of a mould having a moulding space or cavity of said givenshape and dimensions and having walls of uniform thickness duringcooling of metal cast in such mould;

(b) Arranging casting sand or equivalent material to form a cavity thecross-sectional peripheral size and shape of which is substantially thesame as a selected one of the isotherms of said isothermal pattern;

(c) Erecting centrally within said cavity a core of moulding sand orequivalent material corresponding in size and shape to said mouldingspace; and

(d) Casting molten metal in said cavity and around said core; wherebyduring cooling of the metal heat passes at a substantially constant ratefrom the metal to said core.

The various features of the invention Will now be described, withreference to casting moulds in the form of ingot moulds and withreference to the accompanying drawings in which:

FIGURE 1 shows in more or less diagrammatic form a side view of aconventional square ingot mould;

FIGURE 2 shows a cross-section of the mould of FIG- URE 1;

FIGURE 3 shows in more or less diagrammatic form a side view of an ingotmould embodying the present invention;

FIGURE 4 shows a cross-section of the mould of FIGURE 3;

FIGURE 5 shows a cross-section of a conventional rectangular ingotmould; and

FIGURE 6 shows a cross-section of another ingot mould embodying thepresent invention.

Isothermal lines (i.e. lines joining points of equal temperature) aresuperimposed on FIGURES 2, 4 and 5. To avoid confusion, the usualhatching lines denoting a section have been omitted from these figuresand also from FIGURE 6.

Conventional ingot moulds, that is to say moulds designed in accordancewith the usual practice prevailing at the present time, have an externalshape corresponding substantially to that of the mould cavity. Inparticular, a cross-section taken through the middle of a mould having asquare cavity will have an outer perimeter which is generally square andsymmetrical with the cavity.

Such a conventional mould is shown in FIGURES 1 and 2, having asquare-shaped moulding space or cavity 1 and an outer perimeter 2 whichis also generally square, as seen in cross section. The actual shape ofthe perimeter 2 is such that the thickness of the mould wall issubstantially uniform. Lines 3, 4 and 5 represent isotherms within themould wall obtained during careful measurement of the temperaturedistribution ten minutes after steel had been cast in the mould to forman ingot. These isotherms 3 to 5 are in decreasing order of temperatureand it will be seen that the regions of highest temperature bounded bythe isotherm 3 (parts of which extend through the cavity and are notshown) are located along the sides of the mould cavity while the cornersof the cavity are cooler and at the next lower order of temperatureindicated by the isotherm 4. Furthermore, these isotherms indicate apattern of heat flow which continues through the wall of the mould sothat the corners of the mould are at an appreciably lower temperaturethan the sides and the temperature gradient is greatest at the corners.

During experiments, temperature measurements were made after only fiveminutes from casting and the isotherms obtained followed a similarpattern to those above described. Thus, it has been discovered that therate of heat transfer from the ingot to the mould is greatest at thecorners and least through the side walls. It will be hoted thatisothermal lines for temperatures below that of isotherm 5 will emergethrough the mould outer surface at the corners of the mould. Thus,stresses due to temperature differentials are set up in the cornerregions of the mould wall and these cause the cracks which areexperienced in practice in these regions in conventional moulds.

The present invention takes account of the heat distribution patternrevealed by the above-described measurements and provides an ingot mouldin which the wall thickness is not uniform but varies so that the outerperimeter of the mould cross-section follows to some degree the shape ofthe isotherms of a conventional mould. One embodiment of the inventionis shown in FIGURES 3 and 4 in which the cavity 11 is the same sizeandshape as that of cavity 1 but the outer perimeter 12 resembles theshape of the isotherm 5. Thus, the mould wall is thickest at the centreof each cavity side and thinnest at each cavity corner. With this designof mould wall, the varying thickness compensates for the otherwiseuneven heat flow from the cavity and an even heat distribution in themould results. Consequently, the temperature differentials set up in themould wall are considerably reduced relative to those set up in aconventional mould wall and susceptibility to cracking is reduced if notcompletely removed.

Temperature measurements were made in the mould shown in FIGURES 3 and 4at different times after the casting of an ingot therein. Lines 13 to 15represent isotherms obtained from measurements takenten minutes aftercasting. It will be noted that these isotherms are lines extendingsubstantially parallel to the sides of the cavity and it will beapparent, without further explanation, that the heat flow issubstantially uniform throughout the wall thickness.

Whilst the invention has been described with reference to moulds havinga square cavity in section, it is to be understood that it applies to amould of any shape or cavity. FIGURE 5 shows a cross-section of aconventional mould having a rectangular cavity surrounded by a wall 21of substantially uniform thickness. During cooling of cast metal in thecavity, an isothermal pattern is created the shape of which is indicatedby the isotherms 22 to 26. Accordingly, one possible outercrosssectional shape of a mould embodying the invention is substantiallythat of the isotherm 26, as shown in FIG- URE 6 in which the cavity 27is the same size and shape as the cavity 20.

To determine the cross-sectional perimeter and peripheral shape of thewall of a mould in accordance with the invention, the shape and size ofthe cavity or moulding space having been chosen, the isothermal patternwhich exists in a conventional mould is first determined. One convenientmethod of doing this is to manufacture a conventional trial mould havingwalls of uniform thickness. Metal is then cast in the mould and theisothermal pattern (as hereinbefore described) is determined duringinitial cooling of the metal. A mould is then constructed having anouter shape, or outer cross-sectional periphery corresponding withinpractical limits to a chosen isotherm.

Choice of isotherm will depend upon the structural strength requirementsfor the mould which will determine the minimum thickness of the thinnestpart of the wall. Consequently, the isotherm which defines said minimumthickness will normally be chosen. That is to say, referring again toFIGURE 1, the isothermal pattern shows that the thinnest part of themould wall should be at the corners of the cavity. Thus, the minimumthickness is determined for the wall at those corners and theappropriate isotherm (in the aforedescribed embodiment, isotherm 5)determines the shape and dimensions of the wall periphery. It has beenshown by experiment that, for moulds having the same cavity shape but ofdifferent sizes, the external shape remains the same with correspondingdifference in the external dimensions. Thus, the isothermal pattern fora given cavity shape need only be determined once.

As viewed from another of its aspects, the present invention provides anovel method of constructing a mould; eg an ingot mould. Thus, thecasting sand or equivalent material in which the ingot mould is to becast is formed into the usual central core. However, the wallssurrounding the core are formed into a cavity having a cross-sectionalperipheral size and shape substantially the same as one of the isothermsof the aforedescribed isothermal pattern which exists in an equivalentconventional mould. When the metal for forming the ingot mould ispoured, heat flows from this cast metal to the core sand and at asubstantially constant rate throughout the cast metal, during coolingthereof. Experiment has shown that this even distribution of heat flowresults in a reduction, and in some cases the complete avoidance, ofcorner shelling.

Although reference has been made mainly to ingot moulds it is envisagedthat the various features of the invention will be applicable to thecasting of any metal, in any form of mould, and it is intended that theappended claims should be interpreted accordingly.

It is to be understood that the moulds shown in FIG- URES 1 and 3 aremore or less diagrammatic and do not show any particular form of bottomor top constructions which may be employed in connection with thepresent invention. Such features are not illustrated or described sincethe present invention is concerned mainly with the body of the mould,which is sufficiently described to disclose the principle underlying thepresent invention.

I claim:

1. -A method of constructing a casting mold for casting metal, the moldhaving a cavity of given cross-sectional shape and dimensions, saidmethod comprising the steps of:

(a) determining the cross-sectional isothermal pattern which exists inthe wall of a trial mold having a wall of uniform thickness and a cavityof given shape and dimensions during cooling of metal cast in such mold;and

(b) constructing a final mold having a cavity of said givencross-sectional shape and dimensions and having a wall the outercrosssecti-onal periphery of which conforms substantially to a selectedone of the isotherms of said isothermal pattern.

2. A method of constructing .a casting mold for casting metal, the moldhaving a cavity of given cross-sectional shape and dimensions, saidmethod comprising the steps of:

(a) providing a trial mold having a cavity of said given cross-sectionalshape and dimensions and a wall of uniform thickness bounding saidcavity;

(b) casting metal in said trial mold and determining the cross-sectionalisothermal pattern existing in the wall of the mold during the coolingof the casting metal; and

(c) constructing a final mold having a cavity of said givencross-sectional shape and dimensions and a boundary wall the outercross-sectional outline of which con-forms substantially to a selectedone of the isotherms of said isothermal pattern.

References Cited by the Examiner UNITED STATES PATENTS Gathnrann 249-174Blage 249-174 Dockray et al 22-200 Larsen 22-200 Passemar 22-193 Petty22-193 10 J. SPENCER OVERHOLSER, Primary Examiner.

E. MAR, Assistant Examiner.

1. A METHOD OF CONSTRUCTING A CASTING MOLD FOR CASTING METAL, THE MOLDHAVING A CAVITY OF GIVEN CROSS-SECTIONAL SHAPE AND DIMENSIONS, SAIDMETHOD COMPRIING THE STEPS OF: (A) DETERMINING THE CROSS-SECTIONALISOTHERMAL PATTERN WHICH EXISTS IN THE WALL OF A TRIAL MOLD HAVING AWALL OF UNIFORM THICKNESS AND A CAVITY OF GIVEN SHAPE AND DIMENSIONDURING COOLING OF METAL CAST IN SUCH MOLD; AND (B) CONSTRUCTING A FINALMOLD HAVING A CAVITY OF SAID GIVEN CROSS-SECTIONAL SHAPE AND DIMENSIONAND HAVING A WALL THE OUTER CROSS-SECTIONAL PERIPHERY OF WHICH CONFORMSSUBSTANTIALLY TO A SELECTED ONE OF THE ISOTHERMS OF SAID ISOTHERMALPATTERN.